Modern electronic devices often incorporate densely populated circuit boards, very large scale integrated (VLSI) chips, and other semiconductor devices and support elements such as resistors, inductors, capacitors, and connectors. For proper operation all of the devices must reliably electrically interconnect using electrical contacts.
While the foregoing scheme has proven highly successful, it has issues. For example, as VLSI and other semiconductor devices have evolved they have become extremely densely populated with internal circuitry, an evolution that tends to increase the number of input/output (I/O) lines required per device. While increasing the number of I/O lines can and has increased the data transfer and processing speeds of devices, it is disadvantageous because as the number of I/O lines increase more electrical contacts are required per unit board area. This mandates smaller electrical contacts, reduced spacing between electrical contacts, and smaller conductors.
Prior art semiconductor devices were comprised of one or more silicon dies that were fabricated with numerous transistors, gates, and metallic pads that were bonded to a substrate. The substrate had pin connectors that passed through the substrate's outer periphery. Internally, the pin connectors usually had a flat bond area. Fine wires where wire bonded between metallic pads on the silicon dies and the flat bond areas of the pin connectors. The substrate was then attached to a circuit board by having its pin connectors electrically connect to circuit board conductors. While very successful, this technique is inherently limited because only a limited number of pin connectors can be placed around the substrate. As circuit densities increase the inherent placement limitation became a serious problem.
Over the years various approaches have been taken to increase the number of electrical contacts per silicon dies. Many of those approaches had serious problems in aligning the electrical contacts of the silicon dies with the electrical contacts of the circuit boards substrates. However, one highly successful approach to increasing electrical contact density that avoids undue alignment problems is the so called flip-chip approach that uses Controlled Collapse Chip Connections, also known as C4.
C4 electrical contacts use “solder bumps,” also known as “solder balls,” that are formed on a silicon die's metallic pads. The solder ball pattern is designed to electrically connect to similarly positioned flat contacts on a substrate, circuit board, or other die receiver.
After the solder balls are formed a silicon die that die is flipped over and positioned on the flat contacts. The flat contacts, silicon die, and solder balls are then heated to melt the solder. After the solder cools a reliable electrical and mechanical bond is ideally formed between the solder balls and the flat contacts. This description is highly simplified in that the complex and important steps of placement, cleaning, flux use, heating profiles, cooling profiles, and solder compositions are ignored. However, when properly implemented the flip chip approach with its C4 balls have proven extremely useful in increasing electrical contact densities.
Using C4 ball electrical connections have several major advantages beyond the increased number of electrical connections that can be implemented per unit area. One is that the high surface tensions of solders make C4 electrical connections self-aligning. If a semiconductor device is not perfectly aligned with the flat contacts, as the solder melts the solder surface tension causes physical movement of the silicon die that causes the solder bums and flat contacts to align.
C4 solder balls are typically about 100 microns in diameter. While very small, decreasing the solder ball size would enable a further increase in the number and density of electrical connections. However, as the solder balls get smaller electrical reliability problems increase. One major set of problems result from the thermal environment that C4 solder balls can experience during the lifetime of the silicon die. Thermal cycling creates mechanical stresses because of Coefficient of Thermal Expansion (CTE) mismatches between substrate materials, receiver materials (circuit boards, other substrates, etc), silicon dies, metallic pads, flat contacts, and solders, all of which induce stress cracks that can and have lead to device failure.
Another temperature related failure mechanism when using C4 solder ball electrical connections is a result of electromigration. Electromigration is the transport of material by conductor ions due to momentum transfer between conducting electrons and metal atoms. As C4 balls get smaller the electron current densities in the C4 balls increase, which increases metal transport from the C4 balls, which results in various failure mechanisms (metal thinning and voiding, amalgamations, fracturing, hillock failures, etc).
Both electromigration and thermal cycling failures are exacerbated by high temperatures and temperature cycling. Such thermal stress induced failures increase as temperature changes rates increase, the temperature change span increases, and as the number of thermal cycles increase.
Numerous approaches have been made to reduce and hopefully eliminate thermally induced failures of C4 bonds. One approach is to increase the diameter of the C4 bonds, but this decreases and limits the number of possible I/O connections, which is contrary to why flip-chip bonding with its C4 bonds are being used. Other approaches, such as incorporating heat sinks, inserting an additional connection layer to relieve CTE mismatches, attempting to better match CTEs of the various materials, have usually proven unsuccessful either because of additional fabrication complexity; an increase in the number of connections, each of which has its own failure rate, fails to improve reliability; or suitable materials are either unavailable, too costly, or are not conducive with current fabrication technology. Thus the thermally induced failure problems of flip-chip C4 solder ball contacts have not been solved.
In view of the foregoing what is needed is a system and a method of reducing thermally induced failures when using C4 solder ball contacts. Beneficially, such a system and method would not involve increasing C4 solder ball contacts diameters or adding more connections. Even more beneficial, to leverage existing technology and materials, preferably the system and method would not require either new or additional fabrication steps or materials.